This invention relates to a process for producing a sheet molding compound which is in the B-stage and is very easy to handle. More specifically, this invention relates to a process for producing an easily handleable sheet molding compound from a resin composition comprising an unsaturated polyester, a polymerizable monomer and a polyisocyanate compound and glass fibers.
Glass fibers impregnated with unsaturated polyester resins are used in fabricating a variety of articles such as fishing boats, bath tubs, tanks, pipes, containers, and chairs. In recent years, methods have been widely used for producing desired molded articles by using molding materials (so-called prepregs) which are free from the stickiness of resin-impregnated glass fibers. Among them, sheet molding compounds (to be referred to as "SMC") are in widespread use because of their high molding efficiency and freedom from pollution of the working environment. SMC is a B-stage resin-impregnated glass fiber sheet which is obtained by rendering the resin semi-solid (pre-gel) by forming a metallic bond between the carboxylic acid in the unsaturated polyester and an oxide of an alkaline earth metal such as MgO and CaO or a metal hydrate.
Since, however, this metallic bond is susceptible to water, the rate of solidification of the resin and the hardness of the resulting compound differ depending upon the water content of the system. Moreover, the resulting compound must be aged for more than 24 hours to remove tackiness so that it can be used in molding. This leads to the inconvenience that molding conditions should be made to correspond to the hardness of a given compound. Moreover, because a metallic compound is added, a high level of electric insulation cannot be imparted to the compound, and therefore, the compound is unsuitable for use in applications involving high voltages.